1. Field of the Invention
The present invention relates to a positioning method for use in manufacturing a semiconductor device and a projection exposure apparatus using the method, and more particularly, to a positioning method for use in manufacturing a highly integrated semiconductor device, which method aligns, with high precision, a wafer and an image pickup means in synchronization (i.e. alignment) with a reticle in conjugate connection with the wafer by utilizing a coherent image generated based on reflected and diffracted light from a periodic grating mark (wafer mark, alignment mark) formed on the wafer.
This invention also relates to a method and apparatus for detecting the position of an object for use in its positioning.
2. Description of the Related Art
In recent years, semiconductor device manufacturing technology has progressed remarkably, and so has micromachining technology associated therewith. In particular, optical machining technology has reached the level of micromachining technology having a submicron resolution since the manufacturing of one megabyte display random access memory (1MDRAM) semiconductor devices began.
As microminiaturization of ICs (integrated circuits) and LSIs (large-scale integrated circuits) has advanced and the degree of integration of these semiconductor devices has increased year after year, the tolerance of alignment of a mask pattern and a photosensitive substrate pattern has become more exacting. In a well-known method, a reticle and a photosensitive substrate, such as a wafer are aligned with each other according to positional information on the wafer. Various methods of observing an alignment mark on the wafer have been proposed heretofore.
The present applicant has proposed, in Japanese Patent Laid-Open No. 5-343291, a method having high measuring power which adopts a coherent image formed based on reflected and diffracted light from a periodic grating mark on the surface of a wafer as a television image and employs phase detection using FFT (fast Fourier transform) processing.
Microminiaturization of semiconductor devices has recently advanced, and it is important for a projection exposure apparatus to align the reticle and the wafer with high precision in order to increase the degree of integration of these semiconductor devices. In order to achieve highly precise alignment by using the television image obtained by the image pickup means, it is necessary to raise the observing and imaging magnification for the wafer surface to be observed.
However, if the imaging magnification increases, the light intensity of a detection signal decreases. For example, when the magnification is doubled to double the measuring power, the light intensity is reduced to half in a line sensor, or to quarter in a two-dimensional CCD (charge-coupled diode). An insufficient amount of light is likely to cause detection failure and this impedes highly precise detection.